I just ordered a new TiVo Premiere 4 from TiVo. From what I understand the Premiere 4 comes with a 500 gb drive but can only record 75 hours of HD programming. My current TiVo HD has a 500 gb drive (I installed this drive myself to increase recording time from 20 hours to 180) but has room to record @ 180 hours of HD programming. Anyone know why the difference? I ask this question because I just ordered a new Premiere 4 and planned to move the drive from my TiVo HD to the premiere? Also, will I be able to simply swap the TiVo Premiere 4's drive with the one from my TiVo Hd?

Any difference you are seeing is a typo or a mistake of some other kind. The amount of hours that you can record on the TiVo Series 3, HD and Premiere are identical given the same source. However, any hour estimate is only that, a rough estimate. Different channels are compressed at different bitrates and resolutions and that directly impacts the amount of hours that are available to you.

As a rough guide, a 320GB hard drive will net you 45/400 HD/SD hours. Moving up to a 500GB hard drive nets you 75/650 HD/SD hours, everything after that is linear. So a 1TB hard drive will get you double that, 150/1350 HD/SD hours and a 2TB hard drive will double that and put you at 300/2700 HD/SD.